1. Field of the Invention
Embodiments of the present invention generally relate to an emitter for an ion source and, particularly, an emitter for a liquid metal alloy ion source (LMAIS).
2. Description of the Related Art
Focused ion beam (FIB) technology is an important tool for the semiconductor industry. Focused ion beams are used for failure analysis, transmission electron microscopy specimen preparation, and circuit and mask modification. FIB micro and nanofabrication may be utilized to reduce the complexity required in conventional fabrication technology, such as lithography, etching and implantation, which may need to satisfy various requirements for different components fabricated on the same substrate.
The success of FIB technology is due to the invention of liquid metal ion sources (LMIS). In an LMIS, a tiny hairpin and a filament, typically made of W, Ta, Ti or Ni, are typically used as an emitter, such as emitter 610 shown in FIG. 6. Other types of emitters, such as reservoir type emitters with and without hairpin and porous emitters are also known. For example, emitter 620 of the capillary type is shown on the right hand side of FIG. 6.
The emitter is typically wetted and loaded with a source material. For wetting, as well as during operation of the source, the source material is typically provided in liquid form. To this end, a resistance heater or an electron beam heater may be used. High voltage is then applied between the emitter tip and a counter electrode. Due to the high electric field strength at the emitter tip, an even smaller tip of liquid source material forms at the emitter tip and ions are emitted therefrom. Thereby, a stable ion beam is generated from the source material.
Typically, the source material has to fulfill several physical and chemical requirements. It is important, therefore, that the source material is metallic, has a relatively low melting point and a low vapour pressure. It is also important that the source material has good wetting properties with respect to the material of the filament.
Rare earth metals are widely used for laser and optical communications applications. Among the rare earths, praseodymium (Pr) has drawn a lot of interest because of the emission at 1.3 μm, which corresponds to a wavelength for minima in absorption and dispersion in silica optical fibers. Furthermore, Pr is used in Si or GaAs semiconductors. Particularly, DE 100 39 327 A1 describes the use of Pr2O3 as a gate oxide in MOSFETs for further reduction of the gate length. Also, implantation of Pr ion in high-Tc superconductors for the formation of Josephson junctions is reported in “Praseodymium alloy ion source for focused ion beam implantation in superconductors” by F. Machalett et al., Rev. Sci. Instrum. 67 (3), 1996.
However, in “A study of the liquid Pr-ion source”, Phys. D, 20 (1987), p. 1302, S. Papadopoulos reports a poor wettability of tungsten by liquid Pr at temperatures near the melting point. In addition, when exposed to air, the liquid Pr rapidly oxidizes to Pr2O3, one of the most refractory substances known.
Therefore, the authors of “Praseodymium alloy ion source for focused ion beam implantation in superconductors”, Rev. Sci. Instrum. 67 (3), 1996, used the ternary alloy Au—Si—Pr as a source material. However, Au is an improper source material for semiconductor industry. Due to its rapid diffusion, Au quickly disperses within the semiconductor material and alters the conductivity in an uncontrollable manner.
In “Rare earth focused ion beam implantation utilizing Er and Pr liquid alloy ion sources” by L.C. Chao et al., J. Vac. Sci. Technol. B 17(6), November/December 1999, a liquid metal alloy ion source (LMAIS) with binary alloys PrPt and PrAg on a tungsten wire is described. However, these source materials may also introduce Pt or Ag impurities into the semiconductor material.
Finally, EP 0 620 582 describes the use of the binary alloy CoPr for a Co ion source. However, the Pr is only used to lower the melting point in this application.
Therefore, a need exists in the art for a new emitter for an ion source and a method for producing thereof.